Electrical interconnection systems require resistance to oxidation and corrosion as well as a low contact resistance. The system can be static or dynamic. One static system is a connector having a socket and an insertion plug to mechanically and electrically join electrical conductors to other conductors and to the terminals of apparatus and equipment. When located in a hostile environment, such as under the hood of an automobile, the connector is subject to vibration, elevated temperatures and a corrosive atmosphere. The connector must maintain low contact resistance following extended operation and multiple insertions.
One dynamic system is a contact to permit current flow between conductive parts, such as a relay switch for telecommunications. The contact must be capable of many thousands of on-off cycles without an increase in contact resistance.
Electrical interconnection systems are usually manufactured from copper or a copper alloy for high electrical conductivity. Copper readily oxidizes and a protective coating is required to prevent a gradual increase in contact resistance. Historically, gold has been the coating material of choice when the contact force is less than 100 grams. Tin has been employed when the contact force exceeds about 200 grams. Either tin or gold is used for contact forces in the intermediate range.
A hard gold coating is formed by adding a trace amount of cobalt to the gold. The "hard gold" is deposited on the surfaces of a copper or copper alloy connector to a thickness of from about 50 to 100 microinches. The gold coated connector is resistant to oxidation and corrosion and exhibits good wear characteristics. Gold is expensive and the price of gold is volatile, so alternatives have been sought. One alternative is palladium alloys.
Palladium is soft and prone to wear. In connector applications, palladium alloys which are harder than palladium metal are preferred. A connector alloy of palladium and zinc is disclosed in U.S. Pat. No. 2,787,688 to Hall et al. and a palladium/aluminum alloy is disclosed in U.S. Pat. No. 3,826,886 to Hara et al. Other palladium alloys for connector applications are disclosed in a paper by Lees et al. presented at the 23rd Annual Connector and Interconnection Technology Symposium and include Pd/25% by weight Ni and Pd/40% by weight Ag. Ternary alloys such as Pd/40% Ag/5% Ni are also utilized.
While exhibiting good wear characteristics and low initial contact resistance, Pd/Ni and Pd/Ag alloys increase in contact resistance following exposure to elevated temperatures due to the formation of nickel oxide and silver tarnish. A gold flash over the alloy is effective in reducing oxidation initiation sites which then creep along the alloy/flash interface.
It is therefore one object of the present invention to provide a palladium based alloy which has a low initial contact resistance and retains low contact resistance after extended exposure to high temperatures. It is a further object of the invention to provide electrical interconnection systems which are either formed from the palladium alloy or coated with it.
It is the feature of the invention that the palladium alloy contains at least one transition metal selected from Group IVb, Vb or VIb of the Periodic Table and is provided as a composite with copper, either by coating or inlay. It is an advantage of the present invention that the palladium alloys are harder than palladium, exhibit good oxidation resistance and have a low contact resistance, both initially and after extended exposure to elevated temperatures.
These and other objects, features and advantages of the present invention will become more obvious to one skilled in the art from the description and drawing which follow.